Wheat Damping in the Flour Mill

Various degrees of automation available in three principal damping systems.

By David Sugden

Wheat damping is the process by which water is added to wheat, and it serves many purposes.wheat, and it serves many purposes.

Damping is carried out to ensure that the final principal product, flour or semolina, ends up with correct moisture content for onward processing, such as baking and pasta and starch manufacture. Damping also conditions all the kernels for maximum milling performance, including extraction, energy consumption, starch damage and so on. Adding water to wheat is highly profitable  if properly performed.

In the past, millers employed a range of devices, many still in use. These included what is known as the washer and whizzer, the damping wheel and similar machines.

The former has lost its appeal because of effluent problems, maintenance difficulties and fresh water usage. In some cases, the washer and whizzer system actually increased the level of bacteria on the grain surface because of build-up in the washer tank. The water wheel damper, although very ingenious and simple, could not be controlled remotely, nor was it able to wet at high capacity.

Modern wheat dampers take up very little space and are easy to adjust. Replacement circuit boards are easy and cheap to carry. Water pipe plumbing parts are straightforward.

The more sophisticated the model, the greater the need for a trained maintenance crew on site. Diagnosing faults is of the essence, and training courses are of great benefit. These are offered by suppliers of the equipment, who are only too pleased to help. It is critical to the fundamental well being of any mill that the damping system is well maintained because of its pivotal role in the business.

Basic Systems

Three principal damping systems are in use today, with many variations possible. Diagram I above is an example of a “basic” wheat damping process.

Wheat enters and runs through the flow switch (1), which exists to cut off the water supply immediately should there be any interruption and vice versa. Wheat proceeds into the screw mixer (2); a number of variations of this element are on the market, from low speed paddle or crescent blades in a “U” configuration through to high speed equipment contained in a tube. The latter will be shorter than the former simply because grain is moistened uniformly and stabilized faster, although maintenance is required more often. The principle of uniform water application and stability leading to consistency is a necessary feature.

Water is injected at (8) from a water tank, which feeds an infinitely variable motorized control valve (3) that is also capable of being set manually in its simplest version. The function of the valve is to add the desired water as measured by the manometer scale (7), which may be a liter continuous dispenser.

All water passes through a water filter (4) and a totalizer (5), also measuring liters for recording purposes. A magnetic or solenoid valve (6) operates by means of a signal from the wheat flow switch (1). The solenoid valve shuts off or opens according to grain absence or presence.

The entire system may be controlled remotely from a programmable logic controller.

Notice that for the system in Diagram I to work consistently, a non-fluctuating source of wheat flow is required because the system has no metering. Constant water pressure, which is vital, is provided from a water tank with a suitably placed ball-cock control (not shown).

Other available features not shown include a suitable water pressure pump. In more sophisticated versions, a spray- or mist-creating nozzle often is used at the water injection point (8). The nozzle can be operated by water pressure alone or boosted by compressed air in order to aid uniform coverage of kernels. Warning alarms for malfunction also can be added.

The process depicted is a classical device in its simplest form. It can add up to 5% moisture depending on the efficiency of the screw mixer (2). It is also cheap. The disadvantage is that it takes no account of the original or final moisture content of wheat, and so variation is not controlled.

Feed Forward Systems

Diagram II on this page is an example of a fully automatic feed forward wheat damping process. It encapsulates a principle of measurement and control not seen in Diagram I, and it has many valid variations.

The main feature is that it is a feed forward device. This means that grain weight, dry or initial moisture content, temperature and density are all measured before damping, and no measurement is made afterward. A microprocessor calculates the amount of water required, and the system adds it uniformly. This process is capable of manual control.

Particular features starting at the top left of Diagram II include the flow detector or switch (1), which feeds a suitable weigher feeder (2). The flow detector operates as shown in Diagram I. The weigher feeder signals the microprocessor, which in turn compensates the proportion of water added so that grain flow variation is not a problem.

The measure box (3) is of fixed volume with excess grain by-passing to the screw mixer (7). The dry moisture measuring sensor (4) detects the moisture content of wheat by capacitance, a cost effective form. Capacitance measurement detects minor changes in electrical current related to moisture.

Density is measured (5) by a small electronic weigher, whose readings also are picked up by the microprocessor. Density is calculated because the volume is known and can be expressed in kilograms per hectoliter. A temperature probe (6) also feeds information to the microprocessor for compensation.

Otherwise, the process operates as in Diagram I, with a motorized water control valve (8), filter (9), totalizer (10), solenoid shut-off (11), manometer (12) and water injection (13).

The microprocessor, as well as the p.l.c., can record instantly and continuously with trends and alarms, or it can be operated manually. Targets and limits can be set. Accordingly, this is a cost effective capacitance process that compensates for variation in raw material. Its only disadvantage is that it does not confirm, after addition, that the correct amount of water has been delivered.

Feed Back Systems

Diagram III, on page 19, an example of a fully automatic feed back process, has that benefit. It is more expensive and more accurate because of the method of measurement.

Passing the flow detector (1) functioning as in the previous examples, the wheat flow continues to a known volume measuring box (2) on load cells (3), which provides signals to the microprocessor on specific weight or density. Excess wheat bypasses to the screw mixer (4), which in this case is inclined and paddle bladed, although the configuration may vary. A microwave moisture and temperature sensing unit (5) on full feed provides signals for calculation, compensation and water addition commands.

The water addition system works as in the previous examples with a suitable variable valve (6), filter (7), totalizer (8), solenoid shut-off valve (9), manometer (10) and water addition injector (11). Water can be added manually.

The microprocessor can feed a p.l.c. with the full range of monitoring, recording and control, as explained in Diagram II. But accuracy with this system is claimed to be 0.25% better than the system shown in Diagram II because the microwave technique measures accurately freshly damped wheat, whereas the capacitance technique does not. Moreover, because wheat moisture is measured after water addition, a constant feed and a weigher feeder are not required, a characteristic known as “feed back.”

Further features that can be added to all three process types include constant-temperature water or steam for year-round use in extreme climates. Bacteria control sanitation can be facilitated by the use of chlorinated water with stainless steel equipment to avoid corrosion.

In both Diagrams II and III, moisture addition of up to 8% in one pass is possible.

The processes in Diagrams II and III are most effective where wheat variation in capacity, moisture, variety, specific weight and/or temperature is greatest. The basic method in Diagram I is particularly effective on feed to first break, where only a small amount of water is added up to 0.75% and initial moisture content and rate per hour are known.

The three diagrams illustrating the main principles of water addition for wheat damping highlight the degree of automation possible. For degrees of accuracy, you “pay your money and make your choice.”